1. Field of the Invention
The present invention relates generally to mixing appliances, and in particular to a rotary gravity mixer for producing a uniform dispersion of finely divided particles throughout an enclosed volume of liquid.
2. Description of the Prior Art
Operations involving the mixing of solid, liquid and gaseous substances occur in a number of important industrial processes. Each industry has developed mixers unique to its own use and has in most cases done this chiefly on an empirical basis, which has given rise to considerable diversification of the mixing equipment employed. The principal object of any mixing operation is to achieve as homogeneous a mixture as possible. In many cases some kind of physical change of the materials concerned is required to take place during mixing, that is heating, cooling, dissolving, aeration, deaeration, change of state (liquid to solid or vice versa), agglomeration, granulation, dispersion (suspension, emulsion), wetting, coloring, change of viscosity, etc. The intimacy or degree of mixing achieved is directly related to the homogeneity of the mixture. Absolute homogeneity would correspond to theoretical perfect mixing; in actual practice only a certain degree of homogeneity, sufficient to fill the requirements of the process concerned, is aimed at. The individual components of a mixer sometimes offer considerable resistance to the attainment of uniform distribution and dispersion in the specified proportions (by weight or by volume). This may be due to difference in the specific gravity or bulk density of the component materials, the action of adhesive or cohesive forces, surface features the particles, etc.
An essential requirement applying to every mixing operation is that both horizontal and vertical flow of sufficient intensity occur and that all the material is moved frequently into the zone of intense mixing action. Stratification, settling and segregation of the material must not be allowed to take place. These phenomena are liable to occur as the result of gravity or centrifugal force and must be prevented by suitable mixing action.
Mixers for various purposes present a wide diversity of types, including (1) flow mixers, which are used in circulating systems for the mixing of visible fluids, the mixing effect being produced by interference with the flow (jet mixers, injectors, turbulence mixers, etc.); (2) paddle mixers, in which one or more blades rotate on a shaft within the container so that the material to be mixed is moved around in a circular path; (3) propellent mixers, wherein mixing is effected by revolving helical blades which contantly push the material along; and, (4) turbine (or centrifugal and propeller) mixers, which operate on the principal of the centrifugal pump, wherein the material is accelerated by the impeller vanes and is discharged tangentially. The foregoing classes are referred to as positive action mixers, characterized in that a power driven mixing element moves or rotates within a stationary container.
A different type of mixing equipment is the so-called gravity mixer, in which the container is constantly rotated, so that the material inside is tumbled about. The interior of the container may be fitted with lifting scoops or similar devices which lift the material a certain distance and let it fall, thereby intensifying the mixing action. Gravity mixers of this type are sometimes employed for the mixing of material which must not be subjected to the severe mechanical stresses exerted by the mixing elements of positive action mixers as discussed above.
Developing lacquer is an example of a preparation which must be mixed prior to use for best effect. This preparation is generally made from solvents, pigments, and gum solutions and is used for producing printing plates. This mixture forms a colloidal system in which the pigment in the dispersed phase is uniformly distributed in a finely divided state throughout the solvent vehicle, referred to as the dispersion medium. If the lacquer is allowed to stand in a container over a long period of time, the pigment will tend to settle out under the influence of gravity and form a heavy concentration near the bottom of the container. It is therefore necessary to thoroughly mix the lacquer preparation prior to use in order to insure that the pigment is uniformly distributed.
Developing lacquer is commonly supplied in small jug containers which are not suited for receiving the mixing elements of positive action mixers. Positive mixing usually results in air being entrained in the preparation even if it is thoroughly mixed. There is, therefore, a continuing interest in providing a mixing appliance for mixing liquid such as developing lacquer in small containers without introducing air bubbles during the mixing action.
Additionally, preparations such as developing lacquer usually include solvents and gum solutions which are affected by exposure to air. The solvents are very volatile and evaporate easily. Therefore, it would be desirable to provide a mixing appliance for mixing such liquid preparations in a sealed container without introducing air bubbles, and without exposing the liquid preparation to air.
The foregoing and other objects, features and advantages of the present invention will be more fully understood by reference to the following drawings, specification and appended claims.